Bicycle saddles take the brunt--of complaints from uncomfortable riders and of more than half a cyclist's weight. They also rank as a high-wear component. Consider a seat's scuff-prone vulnerability should your bike hit the ground, or the breakdown caused by your sweaty derriere sliding around on it for hours. Like shoes, a saddle is a personal-fit item that needs to be tried on for size. So before you start sampling, here's the lowdown on the parts that make up these posterior perches.

SHAPE:
Different shapes abound to suit many body types and uses. The ischial tuberosities, or sit bones, of females are generally more widely spaced than those of males--hence women-specific saddles are wider. Gender aside, a seat that's too wide will chafe and rub, while one that's too narrow will make you feel like you're straddling a banister. The profile of seats varies as well: Viewed from the front, some are flatter with squarish sides, others curve steadily and are more round. Some companies offer dedicated models for triathlon (thicker padded nose for forward positioning) or off-road freeriding (thick padding overall and a rugged cover). A few saddle manufacturers now even supply shops with special pads to measure your sit-bone width, to help take the guesswork out of picking the right-width seat.

SHELL AND CUTOUT DESIGN:
The hard, structural shell is made from injected-molded plastic, typically nylon. Sometimes carbon fiber is mixed with the plastic to lower weight and tweak flex characteristics, while a few models sport an all-carbon shell for less weight and more zoot. The shell determines how the seat flexes and gives under a rider's weight. In recent years many shells have incorporated holes, slots or grooves through the nose section, all promising additional comfort. "Saddles with a cutout in the nose work best for about 80 percent of riders by shifting pressure away from soft tissue and toward the ischial tuberosities," says Andy Pruitt, Ed.D., who's done extensive research on saddles as the director of Colorado's Boulder Center for Sports Medicine, and as a consultant for Specialized. "Solid-nose saddles still work best for some, particularly cyclists who naturally sit crooked on their seats." Then there are traditional tensioned leather saddles (such as those made by Brooks) that use a piece of cowhide riveted to a frame on the rails, rather than the plastic sandwich system.

PADDING:
This is what gives a saddle its squish. Urethane foam is most commonly used, along with polymer gels, which have a rubbery, fleshlike consistency that offers a reassuring feel to many. Sometimes different padding materials are used on a single seat to add comfort at high-pressure areas. Foams and gels alike are molded onto the shell, with the thickness and density of the padding varying across different models of seats. More padding doesn't mean more comfort. "If your bike fits properly overall the seat can be pretty damn hard," says Pruitt. "Some padding is needed to help disperse that focused pressure point over a slightly bigger area. But when you sit on overly thick padding, it can deform and migrate to places where you don't want pressure, like between the sit bones," says Pruitt. Or, if you want nothing between you and your carbon fiber, there are flyweight saddles with no padding at all.

RAILS:
Until about 15 years ago, virtually all saddle rails were made from cheap, heavy, chrome-plated carbon steel. Then titanium came along and lightened things up, and today remains the material of choice for most high-end seats. Strong, light steels such as chrome-moly, often in hollow tubular form, are also common. Carbon fiber is not only ubiquitous but also easily gouged by sharp seatpost clamps; it's found in the rails of some pricey seats, where it's often wrapped in aluminum for toughness. A few companies use a proprietary monorail or beam system, with a dedicated seat.

COVERING:
The smooth, outer skin can be leather (cowhide, or even exotics like alligator or snake skin), synthetic leather (Lorica), or assorted kinds of fabrics and plastics, including bulletproof Kevlar. Some are perforated with tiny holes, which add a bit of friction to keep you from sliding around. Off-road saddles can take a beating, with the reinforced corners found on some models offering increased durability when bike meets earth. Color options let you stylize your ride, but basic black hides wear and fading best.